Method for the production of 6,6,6-trihalo-3,5-dioxohexanoic acid esters

ABSTRACT

A method for the production of 6,6,6-trihalo-3,5-dioxohexanic acid esters of formula (I):  
                 
 
in addition to the enols thereof and &lt;I&gt;E&lt;/I&gt; and &lt;I&gt;Z&lt;/I&gt; isomers, wherein X independently represents fluorine, chlorine or bromine and R 1  represents alkyl, cycloalkyl, aryl or aralkyl. A method for the production of enol ethers of formula (Ib):  
                 
 
and the enols thereof (E and Z isomers) wherein X and R 1  have the above-mentioned meanings.

This application is a continuation-in-part of the 371 national stage application, filed on Apr. 24, 2006, of International (PCT) Application No. PCT/EP04/011970, filed on Oct. 22, 2004, that has priority benefit of European Patent Application No. 03024344.8, filed on Oct. 24, 2003.

The invention relates to a method for preparing 6,6,6-trihalo-3,5-dioxohexanoic esters of the formula

and the enols and E and Z isomers thereof or the enol ethers thereof, of the formula

and the enols and E and Z isomers thereof in which the substituents X are each independently of one another fluorine, chlorine or bromine, and in which R¹ is in each case alkyl, cycloalkyl, aryl or aralkyl, and R² is alkyl, cycloalkyl, allyl or benzyl, starting from pyranones of the formula

in which X has the abovementioned meaning.

Ethyl 6,6,6-trihalo-3,5-dioxohexanoates of the formula I are employed for example for the production of herbicides and agrochemicals (JP-A-06-049039).

Known methods for synthesizing substituted tricarbonyl compounds having a 3,5-dioxohexanoic ester basic structure start for example from ethyl acetoacetate, which is condensed with ethyl benzoate in THF in the presence of KH/BuLi (WO-A-94/11361), or with a highly substituted 3-oxopentanamide in THF in the presence of NaH/BuLi (WO-A-02/055519).

A method for preparing tert-butyl 6,6,6-trifluoro-3,5-dioxohexanoate from 2,2,2-trifluoroethyl trifluoroacetate and tert-butyl acetoacetate is disclosed in WO-A-02/02547.

A further alternative variant for preparing substituted tricarbonyl compounds proceeds by ring opening of a pyranone such as, for example, of dehydracetic acid, which is converted by means of Mg(OMe)₂ in methanol into methyl 3,5-dioxohexanoate (Batelaan, J. G., Synthetic Commun. 1976, 6, 81-83).

These known methods have the disadvantage that costly reagents such as BuLi are used.

Solladié, et al., Tetrahedron: Asymmetry 1996, 7, 2371-2379, disclosed that ring opening of dehydracetic acid of the formula

to give the tricarbonyl compound is possible, but leads to elimination of the acetyl substituent previously introduced during the synthesis. However, a loss of mass has disadvantageous effects on the profitability of a method in industrial process management.

It was therefore an object of the present invention to provide a simple method for preparing alkyl 6,6,6-trihalo-3,5-dioxohexanoates and the enols and enol ethers thereof, which can utilize easily obtainable pyrones as starting compounds.

This object is achieved according to the invention by the method of the invention.

It has been found that compounds of the formula

in which the substituents X are each independently of one another fluorine, chlorine or bromine, provide, after conversion of the hydroxyl group into an ether group and subsequent opening of the pyran ring with a metal alcoholate, depending on the further reaction conditions, compounds of the formula I or the enol ethers thereof of the formula Ib in good yield.

The present method is distinguished by no loss of mass occurring during the ring opening, and the number of carbon atoms present in the basic structure being maintained.

The method of the invention is surprising because it is known that 4-hydroxypyran-2-one cannot be converted into the open-chain tricarbonyl compound by reaction with sodium methanolate but, on the contrary, as shown in the reaction equation below is firstly methylated on the hydroxyl group and then the pyranone ether is converted into a phloroglucinol derivative (Effenberger, F., et al., Chem. Ber. 1984, 117, 3270-3279).

It was thus not possible to expect the ring opening resulting in the method of the invention. Tetsuro, S., et al., discloses the formation of 6-tribromo-4-methoxypyran-2-one as unwanted byproduct of a bromination reaction of 4-methoxy-6- methylpyran-2-one with N-bromosuccinimide (NBS) in a yield of only 5 percent.

The starting compounds of the formula II of the method of the invention can easily be obtained. Thus, for example, 4-hydroxy-6-trifluoromethylpyran-2-one can be prepared by reacting trifluoroacetic acid with ketene.

Alkyl means here and hereinafter in particular an optionally halogen-substituted, linear or optionally branched group having 1 to 8 carbon atoms, such as, for example, methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl.

Cycloalkyl means here and hereinafter in particular a cyclic group having 3 to 8 carbon atoms, such as, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl.

Aryl means here and hereinafter in particular an optionally alkyl- and or halogen-substituted aromatic group having 6 or 8 carbon atoms, such as, for example, phenyl, p-tolyl or naphthyl.

Aralkyl means here and hereinafter in particular an alkyl group substituted by an aryl group, such as, for example, phenylethyl, where the alkyl group comprises 1 to 4 carbon atoms, and the aryl group is an optionally halogen-substituted, aromatic or heteroaromatic group having 4 to 8 carbon atoms, such as, for example, phenyl, naphthyl, 2- or 3-furanyl, 2- or 3-thiophenyl or 2-, 3- or 4-pyridinyl.

In the method of the invention for preparing compounds of the formula

and the enols and E and Z isomers thereof or the enol ethers thereof of the formula

and the enols and E and Z isomers thereof in which the substituents X are each independently of one another fluorine, chlorine or bromine, and in which R¹ is in each case alkyl, cycloalkyl, aryl or aralkyl, and R² is alkyl, cycloalkyl, allyl or benzyl, compounds of the formula

in which X has the aforementioned meaning, are converted by reacting the hydroxyl group with a compound of the formula (R²O)₂SO₂ or with a compound of the formula Y—R² in which Y is tosyl, chlorine, bromine or iodine, and in which R² in each case has the abovementioned meaning, into a compound of the formula

in which R² and X have the stated meanings, and the pyranone ring of the reaction product is subsequently opened by reaction with a metal alcoholate of the formula R¹O⁻ 1/nM^(n+) in which R¹ has the abovementioned meaning, and M^(n+) is an alkali metal or alkaline earth metal cation and n=1 or 2, depending on the further reaction conditions, to give compounds of the formula I or Ib.

Suitable reagents for the preparation according to the invention of compounds of the formula III are for example dimethyl sulfate, diethyl sulfate, methyl iodide, ethyl bromide, methyl tosylate, ethyl tosylate, phenyl tosylate, allyl chloride, allyl bromide, benzyl chloride or benzyl bromide.

M^(n+) in the metal alcoholates of the formula R¹O⁻ 1/nM^(n+) is preferably Li⁺, Na⁺, K⁺, Mg²⁺ or Ca²⁺.

If a strong acid is added (pH<1) to the reaction mixture of compounds of the formula III after addition of the metal alcoholate, and the mixture is reacted further, compounds of the formula I and enols thereof can be obtained. In this method, the radical R² is eliminated.

If a weak acid, or no acid at all, is added to the reaction mixture of compounds of the formula III after addition of the metal alcoholate, and the mixture is reacted further, enol ethers of the formula Ib and enols thereof can be obtained. In this variant of the method, the radical R² is retained.

The enol ethers of the formula Ib can likewise be converted into compounds of the formula I and enols thereof in poor yields after addition of strong acids and under strongly acidic conditions with elimination of the radical R².

Strong acids mean in the method of the invention for example HCl, HBr, HI, H₂SO₄, trifluoroacetic acid or solid acids such as, for example, acidic zeolites such as H-ZSM-5 or acidic sheet silicates.

Weak acids mean in the method of the invention for example acetic acid and dilute aqueous HCl, H₃PO₄ or H₃SO₄ acids or addition of strong acids after previous addition of water.

In a preferred embodiment, compounds of the formula III are converted into compounds of the formula I in which X is fluorine and R¹ is C₁₋₈-alkyl, with elimination of the radical R². In a further preferred embodiment, R¹ is C₁₋₄-alkyl. In a particularly preferred embodiment, R¹ is methyl.

Compounds of the formula I in the method of the invention also means the corresponding enols such as, for example, of the formulae

singly or as mixture, and are also encompassed by the invention. The equilibrium distribution of compounds of the formula Ia to the enol forms thereof (as E and Z isomers) is influenced by various influences such as, for example, the solvent, the temperature or optionally by protonating or deprotonating additions. After kugelrohr distillation, for example, compound Ia with X=fluorine and R¹=methyl without solvent is predominantly in the form of the monoenol of the formula Ia′ at room temperature.

The enols of compounds of the formula I differ from one another by the enolized carbonyl group and the location and orientation of the resulting double bond(s). The carbonyl groups at C³ and/or C⁵ may be enolized. It is possible in this connection for there to be a double bond in each case between carbon atoms C²/C³, C³/C⁴, C⁴/C⁵ or conjugated double bonds between C²/C³ and C⁴/C⁵, and the double bonds may additionally be in the E or Z configuration. The enols are usually in the form of mixtures of a plurality of forms.

The invention likewise encompasses compounds of the formula:

in which X is in each case independently of one another F, Cl or Br, and in which R² is alkyl, cycloalkyl, allyl or benzyl, with the exception of the compound in which X is bromine and R² is methyl.

Likewise encompassed by the invention are enol ethers of the formula

and enols thereof, such as, for example

in which X is in each case independently of one another F, Cl or Br, and in which R¹ is alkyl, cycloalkyl, aryl or aralkyl, and in which R² is alkyl, cycloalkyl, allyl or benzyl. The compounds of the formula Ib may, just like the compounds of the formula I described above, be in the form of E and/or Z isomers. Depending on the external conditions, however, only the carbonyl group at C⁵ may be enolized. The number and location of the resulting double bonds at C²/C³ and/or C⁴/C⁵ correspond to the E and Z isomers of the enols of the compounds of the formula I.

Alkyl 3,3,3-trihalo-3,5-dioxohexanoates can be prepared by the method described above from 4-methoxy-6-trihalomethylpyran-2-ones. Preferably, methyl 6,6,6-trifluoro-3,5-dioxo-hexanoate is prepared from 4-methoxy-6-trifluoromethylpyran-2-one.

EXAMPLES

The following examples illustrate the procedure for the method of the invention without this being regarded as a restriction.

Example 1 4-Methoxy-6-trifluoromethylpyran-2-one (III; R²=methyl, X=fluorine)

Sodium carbonate (1.35 g, 13 mmol) and dimethyl sulfate (2.17 g, 17 mmol) were added to a solution of 4-hydroxy-6-trifluoromethylpyran-2-one (3.0 g, 17 mmol) in acetone (50 mL). The mixture was heated under reflux for 3 hours and, after cooling, filtered. Concentration of the filtrate resulted in 2.9 g of crude product as a brown oil. It was possible to obtain 4-methoxy-6-trifluoromethylpyran-2-one (2.74 g, 14 mmol, 83%) as colorless needles with a melting point of 61° C. by crystallization from hexane.

¹H NMR (400 MHz, DMSO-d₆) δ: 7.01 (d, J=1.6 Hz, 1H), 5.98 (d, J=1.6 Hz, 1H), 3.7 (s, 3H).

Example 2 Methyl 6,6,6-trifluoro-2-methoxy-5-oxo-2-hexenoate (Ib; R¹=R²=methyl, X=fluorine)

and the enols and E and Z isomers thereof

A solution of 4-methoxy-6-trifluoromethylpyran-2-one (2.7 g, 14 mmol) in a methanolic magnesium methanolate solution (8.5% Mg(OMe)₂, 8.36 g, 8 mmol) was heated under reflux for 16 hours. The solution was concentrated and taken up in water and ethyl acetate, and the organic phase was brought to pH 5 by adding dilute hydrochloric acid. The organic phase was separated off, dried and concentrated. 1.5 g of crude product were obtained as a yellow oil. Kugelrohr distillation at 0.04 mbar and about 160° C. afforded methyl 6,6,6-trifluoro-3-methoxy-5-oxo-2-hexenoate (1.50 g, 6.6 mmol, 48%) as a pale yellow oil.

Data for the main compound: ¹H NMR (400 MHz, DMSO-d₆) δ (resonance lines of the enol form Ib, without E/Z determination): 6.05 (s, 1H), 3.9 (s, 3H), 3.82 (s, 2H), 3.62 (s, 3H). ¹⁹F-NMR (386 MHz, DMSO-d₆) δ: −76.8. MS: 227 [M+H]⁺.

Example 3 Methyl 6,6,6-trifluoro-3,5-dioxohexanoate (I; R¹=methyl, X=fluorine)

and the enols and E and Z isomers thereof

A solution of 4-methoxy-6-trifluoromethylpyran-2-one (10 g, 52 mmol) in a methanolic magnesium methanolate solution (8.5% Mg(OMe)₂, 62.8 g, 61 mmol) was heated under reflux for 2 hours. Concentrated aqueous HCl (25.5 g, 250 mmol) was added to the reaction solution, and the mixture was heated under reflux for a further 2 hours, then cooled and concentrated in vacuo to about 20% of the initial volume. The residue was mixed with 10 mL each of methylene chloride and water. The organic phase was separated off, washed with water, dried over sodium sulfate and concentrated. Kugelrohr distillation at 0.04 mbar and about 160° C. afforded methyl 6,6,6-trifluoro-3,5-dioxohexanoate (2.8 g, 13 mmol, 26%) as a pale yellow oil.

Data for the main compound: ¹H NMR (400 MHz, DMSO-d₆) δ (resonance lines of the enol form Ia′, without E/Z determination): 6.0 (br, 2H), 3.8 (s, 2H), 3.65 (s, 3H).

¹³C-NMR (100 MHz, DMSO-d₆) δ (resonance lines of the enol form Ia′, without E/Z determination): 181.6 (s), 167.7 (s), 116.9 (q, ¹J_(C-F) 286 Hz), 95.9 (t), 52.0 (q), 49.7 (t), C-3 not identifiable. MS: 212 (M⁺). 

1. A method for preparing a compound of formula:

or an enol thereof or a E or Z isomer thereof, in which X is in each case independently of one another fluorine, chlorine or bromine, and in which R¹ is alkyl, cycloalkyl, aryl or aralkyl, comprising (a) initially converting a compound of formula:

in which X has the stated meaning, by reacting the hydroxyl group of the compound of formula II with a compound of the formula (R²O)₂SO₂ or with a compound of the formula Y—R² in which Y is tosyl, chlorine, bromine or iodine, and in which R² is in each case alkyl, cycloalkyl, allyl or benzyl, into a compound of formula:

in which R² and X each has the above mentioned meaning, and (b) converting the compound of formula III by reaction with a metal alcoholate of the formula R¹O⁻ 1/nM^(n+) in which R¹ has the above-mentioned meaning and M^(n+) is an alkali metal or alkaline earth metal cation and n=1 or 2, and (c) further treating with a strong acid, into a compound of formula I and/or an enol thereof and/or an E or Z isomer thereof.
 2. A method for preparing an enol ether of the formula:

or an enol thereof or, in each case, the E or Z isomer thereof, in which X is in each case independently of one another F, Cl or Br, and in which R¹ is alkyl, cycloalkyl, aryl or aralkyl, and R² is alkyl, cycloalkyl, allyl or benzyl, comprising (a) initially converting a compound of the formula:

in which X has the stated meaning, by reaction of the hydroxyl group of the compound of formula II with a compound of the formula (R²O)₂SO₂ or with a compound of the formula Y—R² in which Y is tosyl, chlorine, bromine or iodine, and in which R² in each case has the above-mentioned meaning, into a compound of the formula:

in which R² and X each has the above-mentioned meaning, and (b) converting the compound of formula II by reaction with a metal alcoholate of the formula R¹O⁻ 1/nM^(n+) in which R¹ is alkyl, cycloalkyl, aryl or aralkyl and M^(n+) is an alkali metal or alkaline earth metal cation and n=1 or 2, and (c) optionally further treating with a weak acid into an enol ether of the formula Ib and/or an enol thereof.
 3. A compound of formula:

in which X is in each case independently of one another F, Cl or Br, and in which R² is alkyl, cycloalkyl, allyl or benzyl, with the exception of the compound of formula III in which X is bromine and R² is methyl.
 4. A compound of formula:

or an enol thereof or an E and Z isomer thereof, in which X is in each case independently of one another fluorine, chlorine or bromine, and in which R¹ is alkyl, cycloalkyl, aryl or aralkyl, and in which R² is alkyl, cycloalkyl, allyl or benzyl.
 5. The method in claim 2 wherein conversion product of step (b) is further treated, step (c), with the weak acid into the enol ether of formula Ib and/or the enol thereof. 